This is a 48-week, open-label, AUC-controlled, multicenter study. Sixty children will be enrolled. All subjects will receive DMP 266 and nelfinavir. Concomitant use of nucleoside reverse transcriptase inhibitors (NRTI) will be permitted, but they will not be supplied through this protocol. The initial target AUC for DMP 266 will be between 190 and 380 (M(h. The initial starting dose for children will be 600 mg adjusted for body size. The starting dose for subjects will be adjusted on the basis of the tolerability and DMP 266 plasma concentrations of the first six subjects receiving that dose for two weeks. The target AUC is considered to have been achieved if the dose was tolerated and at least 4 of 6 subjects reached the target AUC. Enrollment of subjects will be ongoing and those begun on a given starting dose will continue on that dose until individual subject dose adjustments are needed according to their individual pharmacokinetics evaluations. Baseline and study efficacy evaluations will include plasma HIV RNA levels, CD4 counts, and viral genotypic and phenotypic resistance analyses. Baseline and study safety evaluations will include the monitoring of adverse experiences, clinical laboratory tests, physical examinations and vital signs. At the end of 48 weeks, subjects will be given the option to continue DMP 266 off-study through a DuPont Merck protocol. Closed to enrollment, new version of protocol extends follow up an additional 52 weeks.